The limitations of dc measurements on doped thin film metal-insulator-metal structures are discussed, particularly in regard to the identification of the band structure of such systems. It is shown that the use of ac techniques provides a much wider scope of measurements which can be used to determine information about the energy diagram of thin film insulators. Furthermore such measurements have prominent characteristics which may be related to the band structure of the sample. The technique has been applied to evaporated Au–MoO3–Au samples, the capacitance of which are found to be extremely temperature sensitive; capacitance changes of 50:1 over 100 °C temperature range are reported. At low temperature the capacitance corresponds to the geometric capacitance, but at high temperatures is independent of the film thickness. The results are explained in terms of Schottky barriers at the Au–MoO3 interfaces, and the energy diagram of the system clarified. The doping density in the MoO6 is estimated to be about 1018 cm−3.